Swift: Extension on [?] to produce [?] possible?

Question

In Swift, I have a custom struct with this basic premise:

A wrapper struct that can contain any type that conforms to BinaryInteger suc

Answer 1

I don't know if there is a simpler solution now, but you can use the same “trick” as in How can I write a function that will unwrap a generic property in swift assuming it is an optional type? and Creating an extension to filter nils from an Array in Swift, the idea goes back to this Apple Forum Thread.

First define a protocol to which all optionals conform:

protocol OptionalType {
    associatedtype Wrapped
    var asOptional: Wrapped? { get }
}

extension Optional : OptionalType {  
    var asOptional: Wrapped? {  
        return self 
    }  
}  

Now the desired extension can be defined as

extension Collection where Element: OptionalType, Element.Wrapped: SomeTypeProtocol {
    var values: [Element.Wrapped.NumberType?] {
        return self.map( { $0.asOptional?.value })
    }
}

and that works as expected:

let arr = [SomeType(value: 123), nil, SomeType(value: 456)]
let v = arr.values

print(v) // [Optional(123), Optional(456)]
print(type(of: v)) // Array>